Low-Altitude Weather: Wind Shear and Microbursts

Weather Considerations for Turbine Pilots

16 CHAPTER

Weather is an especially important topic to turbine pilots because most fly true all-weather operations. Company CEOs authorize corporate airplanes because of their desire for flexible, dependable travel. Charter operators and regional and major airlines earn their money from passengers who expect arrival at their destinations on time, every time. As a result, the turbine aircraft operated by these companies are expected to fly in all but the most threatening weather. Pilots and equipment must be up to the task.

This chapter covers some of the weather phenomena that you’re likely to experience in turbine pilot operations. Weather is a complex and interesting topic and one that deserves your attention in detail far beyond the scope of this book. Truly professional pilots are lifelong students of weather.

Turbine pilots must be expert on both low- and high-altitude weather. Low-altitude weather, due to the approaches and departures that must be made to meet the schedule; high-altitude weather, for enroute travel at turboprop and jet altitudes. Turbine aircraft are most fuel efficient at high altitudes; the ride there is generally smoother, and they are above most of the hazardous weather typically encountered at lower altitudes. High-altitude winds can be very strong and therefore tremendously helpful or problematic, depending on conditions and direction of flight.

Low-Altitude Weather: Wind Shear and Microbursts

By the time you transition to civilian turbine aircraft, you should be pretty knowledgeable about low-altitude weather and associated IFR operations. Therefore, very little of that is covered here. At the same time, you’ve probably not yet been flying in some of the really heavy weather encountered in corporate and scheduled flight operations.

Low-level wind shear and microbursts have been hot topics among turbine pilots in recent years. A string of major air carrier accidents has led to meteorological studies of wind shear associated with fastmoving fronts, downbursts, and microbursts. This section covers some of the characteristics and hazards of these phenomena, along with general procedures recommended by most training departments for successfully dealing with them.

Wind Shear

Wind shear occurs whenever two or more adjacent masses of air are moving in different directions, resulting in a “tearing action” where they meet. Wind shear can occur in horizontal or vertical planes. Low-level wind shear refers to occurrences within 1,000 feet of the ground.

Wind shear can result from any source of shifting winds or vertical air movements. As such, wind shear is often associated with passage of fast-moving weather fronts. Strong winds and uneven heating in mountainous areas are also prime culprits. In each of these cases wind shear may occur in most any type of weather conditions, including clear skies with no associated visible weather.

Microbursts

Wind shear has long been associated with thunderstorms, where powerful updrafts and downdrafts exist side by side. These conditions can easily occupy areas exceeding 15 miles in diameter. Areas of extreme downdraft are sometimes called “downbursts.” Exceptionally dangerous instances of wind shear have been documented with localized downbursts of air known as microbursts (Figure 16.1).

Microbursts are typically associated with very strong thunderstorms. “Wet microbursts” typically occur in the portion of a thunderstorm cell containing the heaviest rain. “Dry microbursts” occasionally occur below virga, particularly in desert and high plains areas. (“Virga” is precipitation that evaporates before reaching the ground.) Look for blowing dust, in these cases, to indicate the presence of downdrafts.

While generally fairly small in size, microbursts are dangerous because of their intensities and the relative difficulty of predicting them. The downdraft of a microburst is typically concentrated in an area of less than a mile in diameter. Vertical velocities within and around them may reach several thousand feet per minute, with localized horizontal outbursts sometimes exceeding 120 knots. The force of a microburst can be incredible. In several recorded cases large circular patches of forest, including trees 2 feet in diameter, have been blown down.

Effects of Microbursts on Aircraft

Wherever a powerful downdraft exists, air striking the ground (within 1,000 feet or more of it) blows radially outward from the core of the downdraft. Horizontal wind shear in the vicinity of a microburst may lead to airspeed changes of 40 knots or more. This can be a real problem for aircraft, especially when flying across a microburst within 1,000 feet of the ground, due to the potential difficulties of recovering from the encounter. Large, heavy turbojet aircraft are particularly vulnerable to microbursts due to their large mass and the “spool up” time required for their engines to produce full power.

The classic scenario for a downburst encounter goes something like this. As an aircraft on an ILS approaches a microburst, it encounters a rapidly increasing head wind due to strong horizontal outflow. This causes the aircraft’s indicated airspeed to suddenly increase, and it destabilizes approach relative to the glideslope. The crew chops power

horizontal vortices cloud bases

rain or virga

outflow

FIGURE 16.1 | Microburst.

microburst downdraft

outflow

ground level

to regain its proper position and airspeed on the ILS but then enters the downdraft. The airplane is now in the unfortunate position of being at reduced power in a severe downdraft. If the crew is fortunate enough to exit the downdraft safely, the airplane immediately enters the area where wind is radiating away from the downburst, now as a tail wind. Indicated airspeed now drops. Losing 30 or 40 knots on approach can be deadly in any aircraft. You can see what a confusing and dangerous situation this combination represents (see Figure 16.2).

Avoidance Procedures

In order to avoid microbursts and associated wind shear, it’s important to analyze all available weather information prior to flight. Monitor frontal activity, since frontal passage indicates probable large changes in wind direction and possible low-level wind shear. Particular attention should be paid to thunderstorms in the departure area and to those forecast for arrival time at the destination airport. Warm temperatures and large temperature-dewpoint spreads are other signs that microbursts could occur. Pay close attention to PIREPS (pilot reports) or controller reports of local wind shear.

If conditions exist for wind shear in any form, monitor the reports of aircraft ahead of you on the approach. ATC may report that “The Lear ahead of you had no problems.” Just remember that, especially when thunderstorms are around, the Lear’s report doesn’t ensure a safe ride.

Microbursts are particularly deadly due to their transitory nature. While typically lasting ten to fifteen minutes, a microburst goes through a definite life cycle. Descending air in the convective cloud mass rapidly develops intensity, plunges to the ground, spreads in a violent burst of outflowing winds, and ultimately dissipates. The danger to an aircraft is greatest when passing through at the moment of greatest intensity, when the downdraft reaches the surface and begins its curl outward.

Because microbursts can develop so quickly, your aircraft could be the one “in the wrong place at the wrong time,” even if the airplane ahead did pass through uneventfully. The problem is especially difficult since there’s little useful correlation between the appearance of a thunderstorm cell, its

1. Aircraft on ILS enters outflow from microburst. Unsuspecting crew notes sudden 30 kt. increase in indicated airspeed, reduces power to stay on glideslope. microburst

2. Aircraft enters microburst core and drops rapidly below glideslope due to combined effects of powerful downdrafts, reduced power, and low airspeed. (Since aircraft is now out of the “headwind” region, its indicated airspeed has dropped 30 kt., in addition to the effects of the power reduction.)

Crew applies max. thrust and pitches up.

FIGURE 16.2 | Effects of a microburst on an aircraft.

glideslope

airport

3. If aircraft is fortunate enough to escape microburst downdrafts, it encounters outflow tailwinds. Indicated airspeed momentarily drops another 30 kt. threatening stall.